In thermal transfer printing successive sections of a donor sheet or web are fed through a linear printing region where they move, in contact with successive lines of a receiver, past a thermal print head comprising a linear array of selectively energizable, pixel-size heater elements. The print head or other means urge the juxtaposed donor and receiver sections into intimate contact at the print zone so that dye is transferred from the donor to the receiver in the pixels beneath energized heater elements of the array. In multicolor thermal printing the receiver is moved through the printing zone a plurality of times so that a plurality of different color image components (e.g. cyan, magenta and yellow) can be successively printed on the donor, in register.
One common configuration for effecting such multicolor printing is described in U.S. Pat. No. 4,745,413, wherein the donor sheet is clamped onto the periphery of a print drum which rotates successive line portions past a linear heater element array. A web bearing successive donor sections of yellow, magenta and cyan dye is fed through the print zone, between the print array and receiver, in a timed relation so that a different color donor section moves through the print zone with the receiver, respectively during each of the print drum rotations.
Although not essential in thermal transfer printing, it is often desirable to heat and compress the dye image(s) on the surface of a receiver sheet. This post treatment, referred to as fusing, seals and stabilizes the dyes of the images and thereby enhances the keeping quality of the print.
Materials such as described in U.S. Pat. Nos. 4,642,655 and 4,804,975 benefit from such fusing, which can include heating of the print to a temperature up to about 240.degree. F. The print materials contain water and/or other liquid components and the vaporization of those liquids during such fusing has, on occasion, caused blister-type defects in the print material.